(1) Field of the Invention
This invention relates to an image exposure device for use in an image forming device like a copier or a facsimile machine, or in an image reading device. More precisely, it relates to a device for exposing a document image and projecting a reflected light therefrom to a light-projected member like a photoconductive drum or a light receiving element through a lens, the device having a function of changing magnification ratios.
(2) Description of the Related Art
When a document image which is exposed by a uniform light amount is projected on a light-projected member like a photoconductive drum through a lens, the projected image has a lower illuminance on its peripheral portion than on its central portion. As a result, an image with a non-uniform illuminance is formed.
To solve this problem, document-exposing members like an exposure lamp and a reflecting mirror have been improved so that a larger amount of light may be irradiated on a peripheral portion of the document image than to a central portion thereof.
However, in a device like a copier which has a function of changing the magnification ratios, the distances between the document and the lens and between the lens and the light-projected member change as a accordance with the magnification ratio. In consequence, even if the document-exposing member is controlled to irradiate a uniform light amount during a same-size mode, a larger amount of light will be irradiated on the peripheral portion than on the central portion in a reduction mode.
U.S. Pat. No. 4,172,658 discloses a copier for solving the above problem in the following manner. A light amount for exposing an image of a document is set so that the projected image has a uniform illuminance during the same-size mode. A shielding board is provided between the lens and the document, and a light from the peripheral portion of the document is partially shielded by moving the board for reduction, whereby the non-uniform illuminance is varied. When this construction, in which light correction is only possible for reduction, is used in a device with a function of magnifying over a wide range of ratios, for example, a function of enlarging and reducing, the document exposing member needs to be controlled to irradiate a uniform light amount at the highest magnification ratio so that the light irradiated on the peripheral portion be shielded by the shielding board at lower magnification ratios for illuminance adjustment. Accordingly, a considerable amount of light should be shielded during low magnification ratios and also in the frequently-used same-size mode (when the device has a function of enlarging and reducing). In a state where a great amount of light is shielded as the above, it requires an exposure lamp with a high light capacity to expose the document properly. This will increase power consumption, heighten the temperatures of the document, the glass document table and other members excessively, and raise manufacturing cost because a higher quality exposure lamp is necessary.
The Japanese Patent Publication Kokai No. 58-68062 discloses a copier in which an amending board is provided near the lens for shielding a light from the central portion of the document partially. In this copier, the amending board is integrally moved with the lens so that the board may keep the same distance with the lens while magnification ratios are changed. Although this construction may restrict, to some extent, the non-uniformity of illuminance caused by changing the magnification ratios, it cannot keep a uniform illuminance at all magnification ratios. Furthermore, since a considerable amount of light is continuously shielded by the amending board, this construction also has the problems that power consumption is increased, the temperatures of the document, the glass document table and other members are excessively raised, and manufacturing cost is increased because a higher quality exposure lamp is necessary.